1. Field of the Invention
The present invention relates to a driving method of a thin film EL display unit and a driving circuit thereof, and specifically it relates to reduction of the withstand voltage of driver ICs employed therein.
2. Description of the Related Art
For example, a thin film EL element of double insulation type (or three-layered structure) is constituted as follows:
As shown in FIG. 5, band-shaped transparent electrodes 2 composed of In.sub.2 O.sub.3 are installed in a parallel fashion on a glass substrate 1, and a dielectric substance 3, for example, Y.sub.2O.sub.3, Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3, an EL layer 4 composed of ZnS doped with an activator such as Mn, and a dielectric substance 3' such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 or Al.sub.2 O.sub.3 like the above-mentioned are laminated in sequence in film thicknesses of 500-10000 .ANG. to form a three-layered structure by the use of a thin film technique such as a vacuum evaporation method or a sputtering method, and thereon band-shaped back electrodes 5 composed of A.lambda. are installed in a parallel fashion in the direction orthogonal to the above-mentioned transparent electrodes 2.
The above-mentioned thin film EL element comprises the EL substance 4 sandwiched between the dielectric substances 3 and 3' between the electrodes thereof, and therefore can be viewed equivalent to a capacitive element. Also, this thin film EL element is driven with a relatively high voltage of about 200V applied. This thin film EL element emits a high-luminance light by an AC electric field, having a feature of long life.
Conventionally, to reduce the modulation power consumption in a display unit using such a thin film EL element, a driving apparatus has been used which provides an N-channel MOS driver and a P-channel MOS driver as a driving circuit of the scanning-side electrodes, and performs field inversion drive which inverts the polarity on a field basis (line sequential drive of one screen). Furthermore, in the U.S. Pat. application Ser. No. 864,509 filed on May 19, 1986 (the counterpart in West Germany is Application No. P3619366.6 filed on June 9, 1986), this applicant provided a driving apparatus wherein a driver IC of push-pull configuration is used on the data side, and the waveforms of the whole pulse voltages of positive and negative polarities applied to picture elements of an EL panel are controlled to eliminate a burning phenomenon due to polarization and thereby the long-term reliability is enhanced, and the power consumption is also reduced.
Description is made on a conventional driving method in reference to FIG. 4. In addition, in FIG. 4, to simplify the matrix structure of an EL panel, for the data-side electrodes, a group of light-emitting picture element electrodes is designated by Xi and a group of non-lightemitting picture element electrodes is designated by Xj. Also, for a group of the scanning-side electrodes, since the EL panel is driven in a line sequential fashion, a lightemitting electrode is designated by Ym, and a group of non-light-emitting electrodes is designated by Yn.
In this equivalent circuit, by turning off switches 28 and 29, all the scanning-side electrodes can be put in the floating state in any state of transistors 25, 26, 25' and 26' in scanning-side driver ICs 30. Next, description is made on a method of applying the modulating voltage. This is classified into the following two kinds of drives. 1.circle. P drive (drive which applies a write voltage positive to the data-side electrodes to the scanning-side electrodes)
Transistors 22 and 23 in a data-side driver IC 31 are turned on and transistors 21 and 24 therein are turned off, and thereafter a switch 27 is turned on. Thereby a current flows from the transistors 23 to the ground through all EL picture elements connected to the group of electrodes Xj, further through all EL picture elements connected to the group of electrodes Xi, and through the transistor 22. Thereby, the potential of the group of electrodes Xi is clamped at OV and the potential of the group of electrodes Xj is clamped at Vm, and an application of the modulating voltage is completed.
By applying the modulating voltage, the potential of the group of electrodes Xi is kept at OV, and the potential of the group of electrodes Xj is kept at Vm. The potential of the scanning-side electrodes Ym and Yn at this time is determined by the ratio of the number of light-emitting picture elements Cb to that of non-light-emitting picture elements Cbn, and the potential is Vs={Cbn/(Cb+Cbn)}Vm.
From this state, the transistor 25 connected to the light-emitting electrode Ym of the scanning-side driven IC 30 is turned on, and the transistor 26 connected thereto is turned off, and simultaneously the transistor 26' connected to the group of non-light-emitting electrodes Yn is turned on and the transistor 25' connected thereto is turned off, and thereafter the switch 29 is turned on, and thereby a positive write voltage Vpd is applied to the transistors 25 and 25'. Resultingly, the voltage Vpd is applied to the group of light-emitting picture elements Cb, and a voltage Vpd-Vm is applied to the group of non-light-emitting picture elements Cbn. Here, the positive write voltage Vpd is equal to a sum of a light emitting threshold voltage Vth of the EL panel (a maximum voltage which does not cause the picture elements to emit light) and the modulating voltage Vm (Vpd=Vth+Vm). Accordingly, the picture elements Cb emit light because of Vpd&gt;Vth, and the picture elements Cbn emit no light because of Vpd-Vm=Vth, and thereby two kinds of states, light emission and non-light emission can be realized. 2.circle. N drive (drive which applies a write voltage negative to the data-side electrodes to the scanning-side electrodes)
The modulating voltage is applied in a manner that "ONs" and OFFs" of the transistors 21, 22, 23 and 24 as described in the P drive in item 1.circle. are changed over, and thereby the potential of the group of electrodes Xi is clamped at Vm, and the potential of the group of electrodes Xj is clamped at OV.
From this state, the transistor 26 connected to the light-emitting electrode Ym of the scanning-side driver IC 30 is turned on and the transistor 25 connected thereto is turned off, and simultaneously, the transistor 25' connected to the group of non-light-emitting electrodes Yn is turned on and the transistor 26' connected thereto is turned off, and thereafter the switch 28 is turned on, and thereby a negative write voltage -Vnd is applied to the transistors 26 and 26'. Resultingly, a potential Vm - (-Vnd) is applied to the group of light-emitting picture elements Cb, and a potential OV-(-Vnd) is applied to the group of non-light-emitting picture elements Cbn. Here, by setting the negative write voltage Vnd equally to the light emitting threshold voltage Vth, the picture elements Cb emit light because of Vm+Vnd&gt;Vth, and the picture elements Cbn emit no light because of Vnd=Vth, and thereby two kinds of states can be realized.
However, in the above-mentioned driving method, during application of the modulating voltage, the potential Vs of the scanning-side electrodes Ym and Yn are varied between OV and Vm depending on the ratio of the number of picture elements of the group of light-emitting picture elements Cb to that of the group of non-light-emitting picture elements Cbn in the EL panel Consequently, in the P drive, when the potential Vs of the scanning-side electrodes Ym and Yn is OV, the positive write voltage Vpd (=Vth+Vm) is applied to the transistors 25 and 25', and a maximum potential difference Vth+Vm is applied to the transistors 25 and 25', and in the N drive, when the potential Vs of the scanning-side electrodes Ym and Yn is the potential Vm, the negative write voltage -Vnd (=-Vth) is applied to the transistors 26 and 26', and the maximum potential difference Vth+Vm is applied to the transistors 26 and 26', and therefore a driver IC to be used is required to have a very high withstand voltage.